Now this is a post that I'm not going to argue against, because you actually justify your concerns. More so, you don't just blindly say "NOOOO I DON'T WANT IT".
Almost everything I wanted to sat has been stated in one way or another, probably in better words than I can think of. Except:
Will this Curse client require a separate - not Minecraft - login to use?
What happens to our current FTB forum accounts? Will we be transferred to Curse, or removed and have to make a new one?
What type of moderation does this repository have? Will the client be able to add third party repositories in the case that Curse is too... strict? I fear that popular mods may be able to shun newer mods from being added. Or that mods not on the Curse repository will, for lack of better words, die.
Hopefully I'm not too late to this party.
1. It might require a Curse login, but I'd expect only if it integrates with the Curse system somehow (other than getting the mods stored there, which I believe can be done without logging in).
2. I think once the transfer is completed and you try to log in, it'll ask you to either create a Curse account or log into an existing one that will then be bound to your FTB account. Hopefully, you'll keep all your posts and stats.
3.
The repository already exists, so you can check that yourself. Obviously you want to filter the "Yay I made a new mod it adds a block and some obsidian tools
" kind of mods, but distinguishing difficult mods from easy mods can be deceiving. You'd probably need people with a fair amount of experience in the modding area.
As for popular mods shunning other mods, I feel it'll be just like before, just on CurseForge instead of Minecraft Forums. If the new mod has the same features as the old, popular mod it won't get anywhere because people can't be bothered learning a new system. If it brings something new to the table, however, people may wish to switch. You saw this a little with the ore processing systems of IC2 and Thermal Expansion, which was driven somewhat by GregTech.
Mods not on CurseForge could die, but then again CurseForge could become the standard for distributing mods anyway so it wouldn't be a problem. The most important thing in this whole thing is whether enough people switch to CurseForge and use the new client for it to become the new standard for distributing mods.
I know I'm probably going to be flamed for this, but all this happening wouldn't be necessary if Mojang had gone through with their plan to create this modding platform. Instead their not doing this forced other companies into action. I agree completely that this platform is needed, Curse just stepped up to the plate to create it.